An anisotropically conductive sheet is a sheet exhibiting conductivity only in its thickness-wise direction or having pressure-sensitive conductive conductor parts exhibiting conductivity only in the thickness-wise direction when they are pressed in the thickness-wise direction. Since the anisotropically conductive sheet has such features that compact electrical connection can be achieved without using any means such as soldering or mechanical fitting, and that soft connection is feasible with mechanical shock or strain absorbed therein, it is widely used as an anisotropically conductive connector for achieving electrical connection between circuit devices, for example, electrical connection between a printed circuit board and a leadless chip carrier, liquid crystal panel or the like, in fields of, for example, electronic computers, electronic digital clocks, electronic cameras and computer key boards.
On the other hand, in electrical inspection of circuit devices such as printed circuit boards and semiconductor integrated circuits, in order to achieve electrical connection between, for example, electrodes to be inspected formed on one surface of a circuit device, which is an inspection target, and electrodes for inspection formed on the surface of a circuit board for inspection, it is conducted to cause an anisotropically conductive sheet to intervene, as a connector, between an electrode region of the circuit device and an electrode region for inspection of the circuit board for inspection.
As such anisotropically conductive sheets, there have heretofore been known those of various structures, such as those obtained by uniformly dispersing metal particles in an elastomer (see, for example, Japanese Patent Application Laid-Open No. 93393/1976), those obtained by unevenly distributing a conductive magnetic metal in an elastomer, thereby forming a great number of conductive path-forming parts each extending in a thickness-wise direction thereof and insulating parts for mutually insulating them (see, for example, Japanese Patent Application Laid-Open No. 147772/1978) and those obtained by defining a difference in level between the surface of each conductive path-forming part and an insulating part (see, for example, Japanese Patent Application Laid-Open No. 250906/1986).
In these anisotropically conductive sheets, conductive particles are contained in an insulating elastic polymeric substance in a state oriented so as to align in the thickness-wise direction, and each conductive path is formed by a chain of a great number of conductive particles.
Such an anisotropically conductive sheet can be produced by charging a molding material with conductive particles having magnetism contained in a polymeric substance-forming material, which will become an elastic polymeric substance by, for example, curing, into a molding cavity of a mold to form a molding material layer and applying a magnetic field thereto to conduct a curing treatment.
However, the following problems are involved when a conventional anisotropically conductive sheet is used as a connector in electrical inspection of a circuit device having projected electrodes composed of, for example, a solder.
Namely, when an operation that projected electrodes that are electrodes to be inspected of a circuit device, which is an inspection target, are brought into contact under pressure with the surfaces of their corresponding conductive path-forming parts in the anisotropically conductive sheet is conducted repeatedly, permanent deformation by the contact of the projected electrodes with pressure, and deformation by abrasion occur on the surfaces of the conductive path-forming parts in the anisotropically conductive sheet, and so the electric resistance values of the conductive path-forming parts in the anisotropically conductive sheet are increased, and the electric resistance values of the respective conductive path-forming parts vary, thereby causing a problem that the following inspection of circuit devices becomes difficult.
In addition, particles with a coating layer composed of gold formed thereon are generally used as conductive particles for forming the conductive path-forming parts for the purpose of achieving good conductivity. However, an electrode material (solder) forming electrodes to be inspected in circuit devices migrates to the coating layers on the conductive particles in the anisotropically conductive sheet when electrical inspection of a great number of circuit devices is conducted continuously, whereby the coating layers are modified. As a result, a problem that the conductivity of the conductive path-forming parts is lowered arises.
In order to solve the above-described problems, it is conducted in inspection of a circuit device to form a jig for inspection of circuit devices by an anisotropically conductive sheet and a sheet-like connector obtained by arranging a plurality of metallic electrode structures each extending through in a thickness-wise direction of a flexible insulating sheet composed of a resin material in the insulating sheet and bring electrodes to be inspected into contact under pressure with the metallic electrode structures of the sheet-like connector in the jig for inspection of circuit devices, thereby achieving electrical connection with the circuit device that is an inspection target (see, for example, Japanese Patent Application Laid-Open No. 231019/1995).
In the jig for inspection of circuit devices, however, it is difficult to achieve necessary electrical connection to a circuit device, which is an inspection target, when the arrangement pitch of electrodes to be inspected of the circuit device is small, i.e., the pitch of the metallic electrode structures in the sheet-like connector is small. Specifically described, adjacent metallic electrode structures interfere with each other in the sheet-like connector small in the pitch of the metallic electrode structures, whereby the flexibility between the adjacent metallic electrode structures is lowered. Therefore, the metallic electrode structures in the sheet-like connector cannot be brought into contact with certainly with all electrodes to be inspected in a circuit device, which is an inspection target, when the circuit device is such that a surface accuracy of its substrate is low, an evenness of the thickness of the substrate is low, or variations of the height of the electrodes to be inspected is great. As a result, good electrical connection to such a circuit device cannot be achieved.
Even if it is possible to achieve a good electrically connected state to all the electrodes to be inspected, considerably great pressurizing force is required. Accordingly, the whole inspection apparatus becomes a large in scale, and the production cost of the whole inspection apparatus becomes high.
When the inspection of the circuit device is conducted under a high-temperature environment, positional deviation occurs between the conductive path-forming parts of the anisotropically conductive sheet and the metallic electrode structures of the sheet-like connector due to a difference between the coefficient of thermal expansion of the elastic polymeric substance forming the anisotropically conductive sheet and the coefficient of thermal expansion of the resin material forming the insulating sheet in the sheet-like connector. As a result, it is difficult to stably retain the good electrically connected state.
In the case where the jig for inspection of circuit devices is formed, it is necessary to produce the sheet-like connector in addition to the production of the anisotropically conductive sheet. It is also necessary to fix these members in a state aligned to each other. Therefore, the production cost of the jig for inspection becomes high.
Further, the conventional anisotropically conductive sheets involve the following problem.
Namely, the elastic polymeric substance forming the anisotropically conductive sheet, for example, silicone rubber has adhesive property at a high temperature, so that the anisotropically conductive sheet adheres to a circuit device when it is left to stand for a long period of time in a state pressurized by the circuit device under a high-temperature environment. Thereby the work of changing the circuit device after completion of the inspection to an uninspected circuit device cannot be smoothly conducted. As a result, inspection efficiency of circuit devices is lowered. When the anisotropically conductive sheet adheres to the circuit device in great strength in particular, it is difficult to separate the anisotropically conductive sheet from the circuit device without damaging it. Therefore, such an anisotropically conductive sheet cannot be used in the following inspection.